1. Field of the Invention
The present invention relates to a technique for providing a high speed retrieving operation for electric documents even if an available memory capacity is limited.
2. Description of the Related Prior Art
It is not difficult to imagine that more electric documents will continuously increase as more information equipment or apparatus such as PC (Personal Computer), with which those documents are created, are increasingly used. Considering this tendency, there have been more requests for a full text document retrieval system that retrieves target documents for tremendous amount of documents.
In addition, recently, there have been increasingly requested for retrieving particular content included in an electric mail such as a sender and its title, or for retrieving content included in a particular logical structure identified by a particular tag (herein a “tag” denotes a name of a structure, such as “title”, “body”, “author”, “date”, as described later) of XML (eXtensible Markup Language) documents. In order to meet the above requests, there have been developed document retrieval systems for providing a retrieving operation in documents with limiting retrieving range or structures of the document. A scan-type full text document retrieving scheme is included in such a document retrieval system. This scheme scans data stored on disks or memories so as to retrieve documents that a searcher desires (hereinafter referred to as “target documents”). For example, a technique is disclosed in JP 2003-30197A in which whole documents are stored on memories of a document retrieval system, so that a full text document retrieving operation can be provided at a high speed.
However, a document retrieval system usually has a limited memory capacity. It is difficult to apply the conventional technique as disclosed in JP 2003-30197A directly to such a case that a size of target documents to be retrieved is greater than the memory capacity of the system. This case may require additional memories for the document retrieval system so that the memories for the system can accept the size of the documents, or may require an additional retrieving operation for the disks as well as the memories in the system. The requirement of additional memories may increase cost, and the requirement of the additional retrieving operation for the disk may increase time depending on the number of documents to be retrieved for the disks.
In order to solve the above-mentioned difficulties, the present invention provides a high speed document retrieving operation in a case of retrieving documents constituted of structured data even if available memory capacities are limited.